1. Field of the Invention
This invention relates to a process for the preparation of isomeric phthalic dialdehydes from the corresponding xylylene glycols.
2. Discussion of the Prior Art
It is known to prepare terephthalic and isophthalic dialdehydes by alkaline hydrolysis of xylylene tetrahalides in accordance with German (BRD) Offenlegungsschrift (DOS) No. 2,339,086. However, there are many by-products which are difficult to separate.
Terephthalic and isophthalic dialdehydes may be produced by oxidation of xylylene in the liquid or vapor phase with the aid of chromic acid, for example, according to J. Thiele and E. Winter, Ann. 311 (1900), 353, or U.S. Pat. No. 3,597,485. However, the aldehyde yield is very low, and large amounts of chromic oxide sludge are troublesome.
The preparation of terephthalic and isophthalic dialdehydes by reduction of terephthaloyl and isophthaloyl chlorides with hydrogen in the presence of a palladium catalyst (K. W. Rosenmund, Ber. 54 (1921), 2888) is uneconomical because of the large amounts of catalyst required.
The oxidation of xylylene glycols with selenium dioxide according to Ber. 64 (1930), 261, is limited to the laboratory because of the high cost of the oxidizing agent.
Oxidation with nitric acid according to Journ. fur prakt. Chem. 151 (1938), 254, requires large excesses of nitric acid and because of its strongly exothermic nature is difficult to control.
Many aldehydes can be produced on an industrial scale by gas phase dehydrogenation of alcohols with catalysts of copper, silver or zinc compounds. However, the process is essentially limited to low-boiling alcohols such as methanol or butanol, since good aldehyde yields are obtainable only with these. (See Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], 4th ed., vol. 7/1, p. 160).
High-boiling alcohols, and particularly benzyl alcohols, cannot be converted economically by that process. The benzaldehyde yield is low because benzyl alcohols form ethers at the requisite high dehydrogenation temperatures and the aldehydes produced split off carbon monoxide. Moreover, the catalyst is subject to tarring and soon loses its activity.
It has been proposed to carry out the dehydrogenation under vacuum and at temperatures of less than 300.degree. C. in the presence of oxygen. The amount of oxygen must be less than 50 percent of the amount necessary for absorption of the hydrogen susceptible of evolving in the dehydrogenation. (R. R. Davies and H. H. Hodgson, Soc. 1943, 282-284; C. Mouren and G. Mignonac, Compt. rend. 170 (1920), 258-261, Compt. rend. 171 (1920), 652). However, while these measures do increase the yields, because of the low dehydrogenation temperature and the applied vacuum they also result in pronounced dilution of the reaction mixture as well as low space-time yields and therefore make for an uneconomical process.
It is not known whether the last-mentioned process lends itself to the production of dialdehydes from glycols. The dehydrogenation of the two hydroxyl groups of the molecule to aldehyde groups which here is required makes it appear likely that the yields will be poor and that the number of by-products will be substantially increased. The preparation of dialdehydes other than those of simple structure may be expected to pose considerable difficulties.
It is an object of the present invention, therefore, to provide an economical process which can be readily practiced also on the industrial scale and which permits the preparation of phthalic dialdehydes in high space-time yields.